Metal halide high-pressure discharge lamps of this type are used particularly for fiber-optic illuminating systems in medicine (endoscopy) and technology (boroscopy), where light at color temperatures between 4500 and 7000 K. and good to very good color rendition in all color temperature ranges, along with high lighting intensities, are needed.
Low-loss coupling of the light into the fiber-optic bunch necessitates good focusing, or in other words a focusing diameter that is less than or at most equal to the usable diameter of the fiber-optic bunch. To produce a corresponding light spot, the arc core is essentially projected by a reflector or other optical system. If the light emitted by the arc core does not include all the spectral components of the total light emitted by the lamp, then the color rendition property of the focused light can worsen compared with that of the unfocused light. It is therefore highly important, with a view to use in the aforementioned focusing systems, to purposefully find fill ingredients that emit at the hot arc core and not only at the cooler arc edge. Moreover, for good focusing and high light intensities at the entry to the fiber-optic bunch, especially compact lamp dimensions and a very short light arc (only a few millimeters) with maximum light densities (on average, several tens of kcd/cm.sup.2) must be sought.
From European Patent Disclosure EP 0 193 086, to which U.S. Pat. No. 9,686,419, Blook et al. corresponds, assigned to the assignee of this application, metal halide high-pressure discharge lamps with similar short light arcs and correspondingly high light densities are known that produce light with good color rendition properties.
However, their disadvantage is that the fills of these lamps contain cadmium. For the sake of environmental protection, at the end of the lamp life the toxic heavy metal, cadmium, must be returned to the raw material cycle or be properly disposed of, which in both cases involves attendant costs. Moreover, the lamps with a Cd filling have an irritating greenish tinge, and the color location is located above Planckian locus.
It is an object of The Invention to create a metal halide high-pressure discharge lamp that has a very short light arc with a very high light density as well as a color temperature between 4500 and 7000 K. at a color location near the Planckian locus, good color rendition, and especially in combination with a strongly focusing reflector or other optical system, and that attains this object with a cadmium-free fill.
Briefly, the fill of the lamp according to the invention comprises mercury, at least one noble gas and at least one halogen, and metals that form halides, namely dysprosium (Dy), hafnium (Hf), lithium (Li) and indium (In). The fill quantities, in micromoles per milliliter (.mu.mol/ml) of vessel volume, are advantageously between 0.3 and 3 each for Dy, Hf and Li, and between 0.2 and 2 for In.
The metal halide high-pressure discharge lamp is operated at specific arc powers between 100 and 180 W per millimeter of arc length. Given the compact geometrical dimensions of the lamp--very short electrode spacing (a few millimeters) and small vessel volume (a few tenths of a millimeter)--this is equivalent to wall loads of 70 to 120 W/cm.sup.2 of wall area of the discharge vessel. By means of the fill components, according to the invention, of the discharge vessel, mean light densities of 25 to 75 kcd per cm.sup.2 of arc area are attained, which can be focused with the aid of a reflector or other optical system onto a light spot whose diameter is less than 10 mm. The particular value of the invention is that the good to very good color rendition (Ra.gtoreq.75) is preserved even after focusing, and the color location is near Planckian locus, and this is achieved with a fill that does without the toxic cadmium used until now.
Dysprosium, with its multiple-line spectrum, assures a high radiation flux in the visible range of the electromagnetic spectrum and additionally contributes to the continuous spectrum. Hafnium also produces a multiple-line spectrum and moreover reduces the tendency to devitrification, by building up a reinforced halogen jacket on the bulb wall. Because of the high vapor pressure of hafnium halides, the tendency to bulb blackening is also reduced, and consequently the usable light flux during the lamp life is increased.
By means of lithium and indium, the radiation flux especially in the red and blue portions of the optical spectral region is reinforced. Overall, the light emitted has a spectral composition that is quite close to that of Planckian radiation, or in other words has good to very good color rendition properties. Depending on the proportion of fill quantities of the various components, light can be generated with a color temperature between 4500 and 7000 K.
The lamp according to the invention is preferably used in dichroitic special reflectors, which essentially project the inner arc core. By the purposeful selection of the two atomic radiators, lithium and indium, which radiate preferentially in the hot arc core, it is achieved that the good color rendition properties are preserved even at the focal point of this reflector. Moreover, by the use of lithium in combination with hafnium, high color stability is attained; that is, the color temperature varies only slightly over the lifetime of the lamp.
For arc stabilization, the discharge vessel can contain in addition up to 3 .mu.mol of cesium per cm.sup.3 of vessel volume. To maintain the halogen cycle process, iodine and bromine are preferably used in a molar ratio between 0.3 and 1.5. The lamp also contains mercury, in an amount of typically a few tens to a few hundreds of .mu.mol per cm.sup.3 of vessel volume and a noble gas, such as argon, as the basic gas. The fill pressure of the noble gas in the cold lamp is less than atmospheric pressure--typically a few tens of kPa--so that in this case risk-free manipulation is possible. On the other hand, the pressure range is high enough that upon ignition an undesired evaporation of the tungsten electrodes with an attendant blackening of the discharge vessel is largely prevented.
The metal halide high-pressure discharge lamp according to the invention, while preferably used in a reflector securely joined to the lamp, can nevertheless also be used without an integrated joined reflector.